1. Field of the Invention
The present invention relates to a vibration type actuator using a vibration wave for driving.
2. Related Background Art
Vibration wave actuators are currently used in a variety of application fields. In such a vibration wave actuator, a distortion generation element serving as an electro-mechanical energy conversion element for generating a mechanical distortion in response to an electric or magnetic field is mounted on an elastic element to constitute a vibration element. The vibration of the elastic element is converted into a continuous or intermittent mechanical motion, thereby outputting this motion. Among the vibration wave actuators described above, a piezoelectric/electrostrictive actuator using piezoelectic and electrostrictive elements as distortion generation elements is most popular.
Among the piezoelectric actuators each using the piezoelectric element, an actuator called a vibration wave motor (ultrasonic wave motor) can constitute a continuous rotation driving source. For this reason, this actuator is mounted as a driving source in an optical device such as a camera in place of a conventional rotary electromagnetic driving motor.
Various types of vibration wave motors are available. Commercially available vibration wave motors can be mainly classified into the following types:
1) a flat type in which a traveling vibration wave is excited by a flat or ring-like elastic element, and a disk- or ring-like rotor is brought into press contact with the elastic element; and
2) a rod type (so-called pencil type) in which a rotor is brought into press contact with a Langevin type vibration element.
A conventional ring-like vibration wave motor in FIG. 9 is exemplified as such a vibration wave motor. A rotor 21 rotating together with an output shaft 22 is pressed by a coned disc spring 23 against an elastic element 20 constituting a ring-like vibration element together with a piezoelectric element 25.
This typical vibration wave motor has a high cost as one of the important problems. In the traveling wave type vibration wave motor, the high cost is caused by the following reasons.
1) The shape precision of the annular elastic element must be strictly managed (several xcexcm to several ten xcexcm) to match the resonance frequencies of two different vibration modes (or to set these frequencies to come close to each other). For this reason, the elastic element must be machined, and mass production techniques such as forging, powder sintering, or pressing cannot be used.
2) The piezoelectric element is expensive (in particular, a large-diameter vibration element has a large amount of waste material, and post-processing operations such as electrode formation and polarization are required).
3) The piezoelectric element adhesion process is required (since the motor performance greatly depends on adhesion precision, greatest care must be taken for cleaning an adhesion surface, obtaining high surface precision, and determining good adhesion conditions).
A rod-like vibration motor less expensive than the above motor is proposed in place of it, as shown in FIG. 8.
In the vibration wave motor in FIG. 8, a plurality of piezoelectric elements (PZTs) 3 and feed electrode plates are sandwiched between first and second elastic elements respectively having central portions with holes coaxial with the outer diameter. The male thread portion of a shaft 5 extending through these holes threadably engages with an internal thread portion 1e of the first elastic element 1. The elastic elements 1 and 2 which interpose the PZTs 3 and the like between them are fastened by the head of the shaft 5, thereby forming a Langevin type vibration element.
A rotor 8 engaging with a gear 11 and contacting a spring case 9 in the thrust direction, the case 9 incorporating a compression spring 14, the gear 11 rotatably supported by a ball bearing 10, and a motor mounting flange 12 fitted in the ball bearing 10 are disposed around the shaft 5. The motor mounting flange 12 is fixed by a nut member 13. The rotor 8 receives the spring force of the compression spring 14 through the spring case 9 and is pressed against the driving surface of the elastic element 1. The rotation force is transmitted to the gear 11 and output outside the motor.
When an alternating signal serving as a driving signal, e.g., a periodic voltage is applied to the PZTs 3, a driving wave as the synthesis of flexural vibrations is generated on the driving surface of the first elastic element 1. This driving wave frictionally drives the rotor 8.
The rod-like vibration wave motor shown in FIG. 8 can be made compact (the piezoelectric elements can be stacked to allow to obtain a compact motor, and the input power can be supplied to this compact motor). The motor is of a Langevin type in which the piezoelectric elements need not be adhered. This allows low cost structurally. In practice, the vibration wave motor in FIG. 8 is less expensive than the vibration wave motor shown in FIG. 9.
The above motor is still more expensive several times than a compact electromagnetic motor, assuming that equal outputs are obtained from these motors. The vibration wave motors have been used in only the technical fields sufficiently utilizing their characteristic features (e.g., quietness, direct driving, and holding power).
The cost reduction is an indispensable factor for using the vibration wave motors in a variety of fields to the same degree as the electromagnetic motors. This is the most important problem.
One aspect of the invention is to provide a vibration type actuator having a vibration element in which an electro-mechanical energy conversion element is sandwiched between first and second elastic elements using a shaft extending through central portions, the shaft clamping the first and second elastic elements from two sides by a flange portion and a nut.
The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.